3. Severe current depressive episode. Low mood may impact upon accurate cognitive assessment and major depression is therefore listed as a feature which, when present, makes it impossible to reliably diagnose PDD in the Movement disorder Society Task Force PDD Criteria. This will be operationalised using the self-completed Beck Depression Inventory and a cut-off score of 13, as recommended by a recent Movement Disorder Society Task Force report. The BDI score is considered robust in the face of mild to moderate cognitive impairment.